Iowa's Geology

The story of Iowa started long before people ever lived on the land. Iowa’s land has formed through a slow and never-ending process. Geologists believe that earthquakes, volcanoes, floods and ice helped form and change Iowa for over 1 billion years. These processes formed the base for Iowa’s farming success, mining industries and even some art forms.

Waves of Change

Iowa is far from an ocean or vast lake, but water still has played an important role in shaping its land. Water has at least three major roles in geology.

• Water erodes rocks. Rain, ice, waves and creeping glaciers break rocks into smaller pieces.
• Fast moving water carries away eroded rocks and soil as sediments.
• When moving water slows, it deposits the sediments in new areas.

Iowa has been under water in one form or another for much of its history. Ancient seas covered Iowa while it was located near the equator. For millions of years, the continental plate on which Iowa sits has slowly wandered around the globe. At one time the plate split apart, creating a rift. Then it moved back together. Several other times, it crashed into other plates. These crashes caused parts of the land to get pushed up out of the water. These earth-quaking events often formed mountains. The oldest mountain remnants in Iowa are found as Sioux Quartzite in the northwest corner of the state.

When the land was above water, waves and rain worked to wear away the rocks. When plates crashed together again, the land often got pushed back down. While underwater, the land became covered with layers of sediment. Each layer of sediment holds clues to the past. Some layers have fossil crinoids, trilobites or even tree pollen. Most of these layers are buried under newer rocks and soil. But the bluffs that line the Mississippi River in northeast Iowa are an exposed outcrop of ancient sea sediments.

Frozen in Time

Although the plate became more stable as it moved close to its current location, it hasn’t stayed dry for long. Frozen water in the form of mile-thick glaciers creeped across parts of Iowa at least four different times. The glaciers pushed a mix of loose gravel and soil, called till, across Iowa. This glacial till forms the base of Iowa’s rich farming soils.

When the climate warmed, the glaciers melted. Enormous amounts of water and ground up rocks rushed across the land, cutting new river valleys. After the water moved on, the sediments dried. Strong winds picked up the sediments and carried them across the land. Much of Iowa is covered by this wind blown sediment, called loess. But the hills along Iowa’s western edge are among the deepest deposits of loess found anywhere in the world.

Iowa seems pretty stable today. There have been only 12 earthquakes with their epicenter in Iowa in historic times. And polar glaciers are getting smaller, not larger. However, water is still at work in Iowa, eroding the soils and rocks. Humans have increased the rate of erosion by removing plant cover and leaving the ground bare. So right now, some of Iowa’s geologic history and its farming productivity are headed down river to be deposited in the Gulf of Mexico.

Mining Iowa’s Treasures

The rocks and soil of Iowa have provided the raw materials for many occupations. Native Americans found chert in the Nishnabotna River valley in western Iowa. They chipped this sedimentary rock to make arrowheads and spear points. Early European explorers discovered deposits of lead and zinc near Dubuque. These ores were mined and refined.

Coal was mined in southeastern Iowa as early as the 1840s. Most of it was burned to heat homes and businesses. When the railroads arrived in the 1860s, more coal was needed to fuel the trains. In 1880 there were over 200 coal mines in the southern and central part of the state. By 1900 there were more than 400 mines. However, by the 1920s more people were using cleaner burning coal that was mined in Illinois. The coal mines of Iowa shut down.

During the times that Iowa was slowly lifting up from underwater, the salt from the seas became concentrated. These sea salts settled as thick deposits which became gypsum. Gypsum is mined to make plaster, cement and wallboard. A huge piece of gypsum taken from the Fort Dodge area in the 1860s was carved into the figure of a giant. This “fake fossil” was then buried near Cardiff, New York. After it was “discovered,” people paid fifty cents each to see the giant. Although the hoax was soon uncovered, the Cardiff Giant is still on display in a New York museum.

Mining is important for the building industry in other ways too. Limestone quarries are found across the state. Limestone is used as gravel for roads, as an ingredient in cement and lime is used to enrich Iowa farm fields. Buildings around the state feature actual stones including glacial erratics, limestone blocks and Sioux Quartzite.

On a much smaller scale, local artists have mined Iowa’s geologic treasures for use in their work. Andrew Clemens found over 42 colors of sand in the bluffs near his home in McGregor. He used this sand to create pictures in clear glass bottles from 1878–1894. From the 1950s through the 1970s, sculptor Isabel Bloom of Davenport collected interesting rocks she found in the Mississippi River. She used them as accents in her concrete creations. And in 2002 Andrew Goldsworthy used limestone from Stone City to create a huge sculpture at the Des Moines Art Center.

Recreation

Geology is not all about work—it has also created some of Iowa’s most interesting places to play. Iowa’s great lakes in the north central part of the state were formed when huge hunks of ice broke off and melted slowly in one place. Caves with stalactites and stalagmites are common in the limestone rocks near the surface in northeast Iowa. Geode hunters search for the state rock in southeastern Iowa. And some limestone quarries allow fossil hunters to search for specimens at unused portions of their sites.

Geology Today

A lot is known about Iowa’s geologic history, but there is still much to be studied. Water washed away much of the evidence from millions of years ago. Geologists working with the Iowa Geologic Services Bureau and many universities are trying to piece together what happened during that time. Farmers study their soil to determine how to best use it. Homeowners and water districts want information about their water supply and quality. Construction companies examine land before they build on it. Geology always has, and always will play an important role in what Iowans see and do.

Sources:

• Pierre, Amanda. “Artist Lays Groundwork for Des Moines Sculpture.” Des Moines Register (Des Moines). May 1, 2002.
• Iowa Geological Services Bureau.
• Iowa Association of Naturalists. Iowa Geology and Fossils: Iowa Physical Environment Series. Ames, Iowa: ISU Extension Service, 1999.
• Land Between Two Rivers: A Guide for Teachers. Iowa PBS, Johnston, Iowa, 1986.
• Prior, Jean C. Landforms of Iowa. Iowa City, Iowa: University of Iowa Press, 1991.
• Schwieder, Dorothy. Iowa: Past to Present. The People and the Prairie. Second Edition. Ames, Iowa: Iowa State University Press, 1991.
• Troeger, Jack Clayton. From Rift to Drift: Iowa’s Story in Stone. Ames, Iowa: Iowa State University Press, 1983.